Psychologists have a name for this -- reactance. The power of this emotion was captured in a 1976 study comparing the efficacy of two signs meant to curb vandalism in a public restroom. One politely asked users to refrain from writing on the stalls. The other said: "Do NOT write on the walls!" Guess which sign was surrounded by more graffiti?

And if any further confirmation is necessary, check out some of the 100+ comments her essay has prompted so far. Among them, "Poet Doc" writes that:

We're all forgetting one important point here: America is supposedly a free country, therefore I have a right to be as unhealthy as I choose to be.

And "Robert" chimes in that:

As free people we are not subject to what the government decides is behavior that should be favored. Should we tax people who don't graduate from school? Should we tax beef? Fast Food? Motorcycles? Cars that go over 35 miles per hour? All of these are bad for the individuals, and cost the government money to care for us.

Reactions like these underscores why a soda tax -- if it ever came to pass -- might backfire. "In the real world," Iyengar writes, "people's desire to assert their freedom to drink soda may very well trump the disincentive of higher cost."

-- Karen Kaplan

Photo: Consumers see freedom in these bottles of sugary drinks. Credit: David McNew / Getty Images